Corrosion of steel strengthening members in concrete has long been a problem in the art, and has received a great deal of attention. For instance, it is known to coat reinforcing bars with an epoxy coating applied by electrostatic spray guns, and the American Society for Testing and Materials has issued standard specifications for epoxy-coated reinforcing bars and steel, under ASTM designations A 775-81 and D 3963-81, covering deformed and plain steel reinforcing bars with protective epoxy coating applied by the electrostatic spray method. This approach is not without its problems in that the coating thickness is specified as 5 to 12 mils, apparently in order to avoid bond problems encountered at greater thicknesses, and the lesser thicknesses involve problems of integrity or permeability of the coating, exemplified by the ASTM specifications permitting two holidays (pin holes not discernible to the unaided eye) per linear foot of the coated bar. Epoxy coating materials are available on the market for use specifically in coating reinforcing bars. A problem remains, however, in assuring an adequate corrosion-protective coating while maintaining good bonding qualities with the concrete.
Corresponding problems, but of greater magnitude and importance, exist in the case of high strength steel wire and strand used for prestressing concrete (hereafter referred to as PC wire or strand). Strand, of course, is formed by spinning a number of wires (typically six) together around a central core. The magnitude of the problem is exemplified by the fact that use of PC strand or wire is discouraged or prohibited in certain areas where it advantageously could be used. Thus, in a Memorandum dated Feb. 10, 1981, of the Federal Highway Administration, U.S. Department of Transportation, captioned "Corrosion Protection of Reinforcement in Bridge Decks," and dealing with criteria to be applied to all reinforcement in bridge decks, prestressed or otherwise, where deicing salts or a salt water environment present the potential for corrosion, it is suggested that all conventional reinforcement be epoxy coated, but that "Pretensioning should not be permitted in bridge decks since there is no known way of eliminating the potential for corrosion," and that "Polyethylene ducts should be provided for protection of posttensioned tendons in addition to grouting." In a follow-up Memorandum dated Apr. 14, 1981, indicating that epoxy coating of rebars was not intended to be the only method of corrosion protection of bridge decks, it was stated that "In pretensioned work, there are currently no known methods for epoxy coating the strands, and the potential for corrosion exists in a salt water environment as well as in areas where deicing chemicals are used."
There have been efforts to develop corrosion resistant PC tendons, and some are in use because nothing more efficient and/or more economical was available. Thus, the use of galvanized strand is often suggested by designers concerned about corrosion and not familiar with the properties of galvanized strands. Galvanized strands are not as strong as stress-relieved strands of the same size, and they cannot be fabricated so that they possess all of the desirable properties that are obtained by stress-relieving uncoated strands. They are appreciably more expensive per unit of strength, their bond properties are not consistent, and there can be a chemical reaction between the zinc coating and the cement paste in concrete. Although galvanized strands have been available since before the development of prestressed concrete, they are seldom used. Single unbonded posttensioned strands are used in the construction of flat slab floors for garages, apartment and office buildings, etc., and tendons made of several parallel wires are used in a similar manner. These tendons are typically coated with a corrosion resisting grease, encased in tubing, fastened in place and the concrete slab is cast around them. When the concrete has cured the tendon is tensioned and then permanently held under tension by an anchor at each end. At present, tendons of this type are being coated with grease and encased in plastic tubing. This is an improvement on the former paper wrap, but they are still subject to corrosion in the anchorage area because typically the tubing must be removed to permit the anchor to grip the strand. Additionally, the relatively thin plastic is sometimes damaged during handling.
Posttensioned grouted tendons have been in use as long as prestressed concrete itself. The tendon is threaded through a cavity that has been cast in the concrete, or the tendon is encased in an oversized flexible metal or other type of tube before concrete is cast. After the concrete is cured, the tendon is tensioned, and the cavity around the tendon is pumped full of liquid cement grout. The cavity can be filled if the tendon is properly detailed and fabricated, and if the grout is properly injected. In actual practice, this is frequently not the case, and areas susceptible to corrosion are left in the cavity.
In precast pretensioned members, the tendons typically are seven-wire strands which are tensioned and anchored in the forms. Concrete is cast around the strands. When the concrete has cured, the strands are released from their external anchors, and their prestressing force is transferred to the concrete by bond between the steel and the concrete. Thus, in such pretensioned PC tendons, there is a problem not only of corrosion protection, but also one of bond transfer between the pretensioned PC tendon and the concrete.
The patented technology is replete with various approaches to the problems of corrosion protection and/or bonding characteristics, including some incidental disclosures. For instance, Billner U.S. Pat. Nos. 2,319,105 and 2,414,011 mention thermoplastic or thermosetting coverings which will harden and effect a bond between the concrete body and its reinforcement. Simonsson U.S. Pat. Nos. 2,591,625 and 2,611,945 involve coatings including siliceous material. Wijard U.S. Pat. No. 3,030,664 discloses reinforcing elements provided with a coating comprising a suspension of a hydraulic cement and rubber in suitable proportions as to be converted by steam curing of the concrete into a hard strong layer having good adhesion to the reinforcing elements and the concrete, and supposedly serving also as a rust-protective film. Rice U.S. Pat. No. 3,293,811 discloses an epoxy resin coating on PC strand to protect against notching by serrated teeth carried by anchor wedges. Mager U.S. Pat. No. 3,377,757 relates to steel storage tanks prestressed by plastic coated tendons extending about the tank, the plastic coating being for the purpose of protecting the tendons from corrosion forces. Lang U.S. Pat. No. 3,513,609 relates to posttensioned type tendons, including one embodiment in which the tendon incorporates a curable plastic material such as an epoxy resin between the wire or strand and an outer plastic coating, the curable resin being cured while the wire is held under tension so as to anchor the wire to the outer plastic coating and thus to the concrete structure along the length of the wire. The curable resin initially provides a lubricating effect and, after curing, provides a bonding effect. Curing of the resin is by passing electric current through the core wire. Lang U.S. Pat. No. 3,579,931 is of the same substance. Scott U.S. Pat. No. 3,596,330 discloses a structural tensile member made of steel wire or like material provided with a sheath or coating of polypropylene or other impermeable corrosion resistant material. Lang U.S. Pat. No. 3,646,748 discloses a PC strand encased in a corrosion inhibitor, and encompassed by a seamless plastic jacket tightly covering the encased strand. Palm U.S. Pat. No. 3,755,003 discloses concrete reinforcing elements coated with a coating comprising a pulverulent metal in intimate mixture with the residue from a composition containing an organic component plus a hexavalent-chromium-providing substance. The coating is stated to provide corrosion resistance and enhanced adhesion for concrete to the coated element. Kitta U.S. Pat. No. 3,922,437 involves coating of PC strand with an inner resin layer and then with a lubricant-containing thermoplastic material, and mentions increased corrosion protection provided by the inner resin layer.
From the foregoing, it will be apparent that the problems to which the instant invention is directed are long-standing and important, and that various solutions and approaches have been proposed. However, to our knowledge, the solutions and advantages provided by the present invention are not known in or reasonably derivable from the prior art.
Generally in accordance with the invention, there are provided concrete strengthening members, particularly PC tendons having formed thereon a strongly adherent plastic coating which may be substantially impermeable for improved corrosion resistance, and/or which may have embedded therein abrasive or grit-form particles to provide improved bond with the concrete, and particularly to provide controllable bond transfer in PC tendons of the pretensioned type. While the basic thrust of the invention involves improving PC tendons, those aspects whereby an impermeable coating can be obtained while also controlling bond characteristics are considered, applicable also to conventional steel reinforcing bars wire reinforcement for use in pressure vessels, pipe or the like, or other members. The plastic coating preferably is applied electrostatically from an aerated cloud of charged particles of resin powder, and fusion bonded by heat. The abrasive preferably is applied by spraying during a viscous state of the heated resin at a time when the resin has become fusion bonded into an integral coating, and can be varied as to size and spacing density so as to control the surface condition and the bonding effect. Among features achievable by the invention in its various aspects are corrosion resistance under high tension, ductility of strand and coating, adherence of the coating, toughness of the coating, abrasion resistance of the coating, integrity of the coating under stress and bending angles (an important feature because of the packaging of strand in coil packs), controllable bond transfer, and desired coating thicknesses while retaining overriding control of bond characteristics. In keeping with the invention, three forms of improved PC strand may be provided. Thus, there may be provided a corrosion resistant strand designed primarily for posttensioning, utilizing only the plastic coating, where bond transfer is not a consideration. There may be provided also strand having greatly improved corrosion resistance, plus bond transfer characteristics equal to or exceeding bare strand, thus offering readily controllable bond transfer in a strand of good corrosion resistance. Thirdly, where corrosion resistance is not a major consideration, there may be provided strand of relatively modestly improved corrosion resistance, but with readily controllable bond transfer characteristics.
Before discussing exemplary preferred embodiments of the invention, it is believed in order to mention two known concepts or characteristics relating to prestressed concrete members, these being the "transfer length" and the "development length." In a typical precast, pretensioned PC member, the prestressing strands or wires are placed in the empty forms, stretched to a high tension and held at that tension by temporary anchors located beyond the ends of the forms. The forms are then filled with concrete which completely surrounds each tendon. When the concrete has cured to the required strength, the temporary anchors are removed, and the load in the tendon that was carried by the anchors is transferred to the member by bond between the tendon and the concrete. The tension in the tendon at the extreme end of the member is zero. Within the member the tendon is trying to contract to the zero load length that existed before it was stretched. Bond or adhesion between the surface of the tendon and the concrete prevents this. Since the unit strength of the bond between the tendon and the concrete is small with respect to the total load in the tendon, an appreciable length of contact between concrete and tendon is required to transfer the full load from tendon to concrete. The length of contact required to transfer the full load is called "transfer length." "Transfer length" can be defined as the distance from the end of the member to the point at which the full load in a fully bonded tendon has been transferred to the concrete. Transfer length is influenced by tendon size, shape, material and surface condition and by the consistency of the concrete placed around it. Tests on seven-wire strands with diameters up to and including one-half inch indicate no difference in transfer length for concrete strengths of 1700 psi to 5000 psi. The second concept or characteristic is known as "development length." When a pretensioned bonded prestressed concrete flexural member is loaded from its normal working load to ultimate flexural capacity, there is a large increase in the tension in the strand. As the tension in the strand increases, the length of strand required to transfer the tension to the concrete also increases. The length required to develop the tension which exists at the time of ultimate flexural failure is called the "development length." For an uncoated seven-wire strand, the development length is much greater than the transfer length. For a typical one-half inch diameter uncoated strand, the transfer length is computed to be approximately twenty-five inches, whereas the development length is approximately eighty inches. In most cases for a strand that is debonded at the end of the member, the development length becomes 160 inches. The size and number of strands in a particular member are frequently determined by the development length, and a more economical design can be achieved if the development length is shorter. It is probable that a much shorter development length can be obtained with a grit-coated strand in accordance with the invention.